The advent of xerography, and particularly color xerography, has provided the unscrupulous with the means for unauthorized duplication of original documents for the purpose of passing them off, with or without alteration, as an original document of the same kind. The problem is widespread, and well-known to the issuers of such original documentation, with the result that considerable attention has been given to ways and means to prevent the effective duplication of such documents by color xerography.
Out of such development, it has become understood that xerographic copiers have a screen value, or dot frequency, threshold above which the copier is unable to distinguish the individual elements of the dot pattern of halftone printing, and that, as to color xerography additionally, there are spectral ranges of color in which the reproductive capability of the copier is relatively impaired. These phenomena have been employed in various ways by those skilled in the printing art to foil the unauthorized xerographic duplication of valuable documents by causing invalidating indicia of tampering, essentially latent to the naked eye looking at the original document without the aid of magnification, to appear boldly in the xerographic copy.
Although the phenomena which make this form of document protection possible are generally known, the problem faced by all such methodology is to produce indicia of tampering which are truly latent in the original, even to the relatively low threshold of perception of the mere casual observer.
In most systems heretofore developed for the purpose, the indicia of tampering are printed in one dot frequency or screen value and the background in another, and the indicia camouflaged either with an intermediate third dot frequency immediately surrounding the warning indicia, or with a covering overlay of extraneous pattern, intended to confuse the eye sufficiently to render the warning indicia indiscernible to ordinary observation. While straightforward enough in concept, such systems tend to be complicated in execution, leaving a simpler but effective system to be desired.